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><A
NAME="FDW-CALLBACKS"
>50.2. Foreign Data Wrapper Callback Routines</A
></H1
><P
>     The FDW handler function returns a palloc'd <TT
CLASS="STRUCTNAME"
>FdwRoutine</TT
>
     struct containing pointers to the following callback functions:
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>FdwPlan *
PlanForeignScan (Oid foreigntableid,
                 PlannerInfo *root,
                 RelOptInfo *baserel);</PRE
><P>

     Plan a scan on a foreign table. This is called when a query is planned.
     <TT
CLASS="LITERAL"
>foreigntableid</TT
> is the <TT
CLASS="STRUCTNAME"
>pg_class</TT
> OID of the
     foreign table.  <TT
CLASS="LITERAL"
>root</TT
> is the planner's global information
     about the query, and <TT
CLASS="LITERAL"
>baserel</TT
> is the planner's information
     about this table.
     The function must return a palloc'd struct that contains cost estimates
     plus any FDW-private information that is needed to execute the foreign
     scan at a later time.  (Note that the private information must be
     represented in a form that <CODE
CLASS="FUNCTION"
>copyObject</CODE
> knows how to copy.)
    </P
><P
>     The information in <TT
CLASS="LITERAL"
>root</TT
> and <TT
CLASS="LITERAL"
>baserel</TT
> can be used
     to reduce the amount of information that has to be fetched from the
     foreign table (and therefore reduce the cost estimate).
     <TT
CLASS="LITERAL"
>baserel-&gt;baserestrictinfo</TT
> is particularly interesting, as
     it contains restriction quals (<TT
CLASS="LITERAL"
>WHERE</TT
> clauses) that can be
     used to filter the rows to be fetched.  (The FDW is not required to
     enforce these quals, as the finished plan will recheck them anyway.)
     <TT
CLASS="LITERAL"
>baserel-&gt;reltargetlist</TT
> can be used to determine which
     columns need to be fetched.
    </P
><P
>     In addition to returning cost estimates, the function should update
     <TT
CLASS="LITERAL"
>baserel-&gt;rows</TT
> to be the expected number of rows returned
     by the scan, after accounting for the filtering done by the restriction
     quals.  The initial value of <TT
CLASS="LITERAL"
>baserel-&gt;rows</TT
> is just a
     constant default estimate, which should be replaced if at all possible.
     The function may also choose to update <TT
CLASS="LITERAL"
>baserel-&gt;width</TT
> if
     it can compute a better estimate of the average result row width.
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
ExplainForeignScan (ForeignScanState *node,
                    ExplainState *es);</PRE
><P>

     Print additional <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output for a foreign table scan.
     This can just return if there is no need to print anything.
     Otherwise, it should call <CODE
CLASS="FUNCTION"
>ExplainPropertyText</CODE
> and
     related functions to add fields to the <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output.
     The flag fields in <TT
CLASS="LITERAL"
>es</TT
> can be used to determine what to
     print, and the state of the <TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node
     can be inspected to provide runtime statistics in the <TT
CLASS="COMMAND"
>EXPLAIN
     ANALYZE</TT
> case.
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
BeginForeignScan (ForeignScanState *node,
                  int eflags);</PRE
><P>

     Begin executing a foreign scan. This is called during executor startup.
     It should perform any initialization needed before the scan can start,
     but not start executing the actual scan (that should be done upon the
     first call to <CODE
CLASS="FUNCTION"
>IterateForeignScan</CODE
>).
     The <TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node has already been created, but
     its <TT
CLASS="STRUCTFIELD"
>fdw_state</TT
> field is still NULL.  Information about
     the table to scan is accessible through the
     <TT
CLASS="STRUCTNAME"
>ForeignScanState</TT
> node (in particular, from the underlying
     <TT
CLASS="STRUCTNAME"
>ForeignScan</TT
> plan node, which contains a pointer to the
     <TT
CLASS="STRUCTNAME"
>FdwPlan</TT
> structure returned by
     <CODE
CLASS="FUNCTION"
>PlanForeignScan</CODE
>).
    </P
><P
>     Note that when <TT
CLASS="LITERAL"
>(eflags &amp; EXEC_FLAG_EXPLAIN_ONLY)</TT
> is
     true, this function should not perform any externally-visible actions;
     it should only do the minimum required to make the node state valid
     for <CODE
CLASS="FUNCTION"
>ExplainForeignScan</CODE
> and <CODE
CLASS="FUNCTION"
>EndForeignScan</CODE
>.
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *
IterateForeignScan (ForeignScanState *node);</PRE
><P>

     Fetch one row from the foreign source, returning it in a tuple table slot
     (the node's <TT
CLASS="STRUCTFIELD"
>ScanTupleSlot</TT
> should be used for this
     purpose).  Return NULL if no more rows are available.  The tuple table
     slot infrastructure allows either a physical or virtual tuple to be
     returned; in most cases the latter choice is preferable from a
     performance standpoint.  Note that this is called in a short-lived memory
     context that will be reset between invocations.  Create a memory context
     in <CODE
CLASS="FUNCTION"
>BeginForeignScan</CODE
> if you need longer-lived storage, or use
     the <TT
CLASS="STRUCTFIELD"
>es_query_cxt</TT
> of the node's <TT
CLASS="STRUCTNAME"
>EState</TT
>.
    </P
><P
>     The rows returned must match the column signature of the foreign table
     being scanned.  If you choose to optimize away fetching columns that
     are not needed, you should insert nulls in those column positions.
    </P
><P
>     Note that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s executor doesn't care
     whether the rows returned violate the <TT
CLASS="LITERAL"
>NOT NULL</TT
>
     constraints which were defined on the foreign table columns - but the
     planner does care, and may optimize queries incorrectly if
     <TT
CLASS="LITERAL"
>NULL</TT
> values are present in a column declared not to contain
     them.  If a <TT
CLASS="LITERAL"
>NULL</TT
> value is encountered when the user has
     declared that none should be present, it may be appropriate to raise an
     error (just as you would need to do in the case of a data type mismatch).
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
ReScanForeignScan (ForeignScanState *node);</PRE
><P>

     Restart the scan from the beginning.  Note that any parameters the
     scan depends on may have changed value, so the new scan does not
     necessarily return exactly the same rows.
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>void
EndForeignScan (ForeignScanState *node);</PRE
><P>

     End the scan and release resources.  It is normally not important
     to release palloc'd memory, but for example open files and connections
     to remote servers should be cleaned up.
    </P
><P
>     The <TT
CLASS="STRUCTNAME"
>FdwRoutine</TT
> and <TT
CLASS="STRUCTNAME"
>FdwPlan</TT
> struct types
     are declared in <TT
CLASS="FILENAME"
>src/include/foreign/fdwapi.h</TT
>, which see
     for additional details.
    </P
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